1. Field of the Invention
The present invention relates to methods of treating landfills. Specifically, the invention provides a method of injecting steam into a landfill to accelerate the decomposition/biodegradation of organic refuse within the trash prism, which increases the production of methane gas, and increases the rate of settlement of the landfill. The rate of settlement of the landfill is further increased by raising the temperature therein to melt the plastic component of the refuse.
2. Description of the Related Art
In general, landfills are constructed using the xe2x80x9cdry tombxe2x80x9d method, in which the refuse in the landfill is kept as dry as possible both during construction and when the landfill is closed and capped. This method minimizes the possibility of leachate leaking into groundwater and contaminating it. However, dry conditions are not conducive to the decomposition of the organic refuse. Instead, the organic refuse remains dormant for decades until water infiltrates the landfill in an uncontrolled and natural manner. The water infiltration may cause gas migration, which can lead to groundwater contamination.
The slow decomposition of the organic refuse under dry conditions also slows the settling of the landfill and hinders the production of methane gas, which is a natural by-product of anaerobic (oxygen-starved) decomposition of organic material. Delaying the complete settling of a landfill is disadvantageous, because until the landfill settles, the landfill site is not useful for any purpose other than a garbage dump. In addition, methane is useful as a fuel to produce electricity, for example. Therefore, it would be of great benefit to encourage the rapid decomposition of the organic component of the landfill in order to more efficiently capture the methane produced thereby.
Moisture accelerates decomposition of organic refuse, but does not accelerate the decomposition of the non-organic refuse. Thus, addition of moisture to the trash prism increases the purity of methane extracted from the landfill, because the proportion of decomposing organic refuse to decomposing inorganic refuse is higher as compared to a dry trash prism. The extracted methane is thus more useful because it has a higher Btu value. If the refuse is flooded with water, however, the gas becomes bound up in the liquid and is difficult to recover. Further, introducing water after a landfill has been closed cools the refuse. But decomposition proceeds best at a temperature around 100xc2x0 F. Therefore, a method of introducing moisture into a trash prism that does not flood the trash prism or cool the trash prism would be of great benefit to the landfill-management industry.
One useful method of monitoring conditions within a landfill is a piezo-penetrometer test (PPT) profile. A PPT is an instrument having sensors that measure several parameters within the landfill as the instrument is hydraulically pushed into the landfill. Parameters such as soft and dense layers, vacuum, and gas and liquid pressure are recorded in a computer. This data is then used to develop a three-dimensional profile of the in-situ conditions within the landfill.
PPT profiles of landfills have shown that liquids tend to collect on top of dense and daily cover layers inside landfills, and that gases collect underneath these layers. Dense and daily cover layers are the component of the landfill that is added at the end of each day during the active phase of the landfill. The refuse deposited into the landfill each day is covered by a layer of dirt or a suitable dirt alternative. The non-uniform distribution of liquid around these layers only causes the biodegradation of the organic material in the immediate area of the liquid, rather than throughout the entire trash prism. Thus, a method of evenly distributing moisture throughout the trash prism would greatly enhance the biodegradation of the organic material in the landfill.
U.S. Pat. No. 5,695,641 to Cosulich et al., discloses a method and apparatus for enhancing methane production in a landfill. The method comprises injecting ammonia into the landfill to thereby reduce residual oxygen levels, provide a rich source of nitrogen nutrient for the anaerobic microbe population and increase the pH. The ammonia is injected via injection wells, and may be injected in any form, diluted by a non-oxidizing carrier gas or in aqueous form. The Cosulich method, however, does not suggest the benefits gained by increasing the moisture content of the landfill, or address the detrimental effects of lowering the temperature of the landfill by introducing water.
U.S. Pat. No. 6,024,513 to Hudgins et al., discloses a method of decomposing municipal solid waste (MSW) within a landfill by converting the landfill to aerobic degradation in the following manner: (1) injecting air via the landfill leachate collection system; (2) injecting air via vertical air injection wells installed within the waste mass; (3) applying leachate to the waste mass using a pressurized drip irrigation system; (4) allowing landfill gases to vent; and (5) adjusting air injection and re-circulated leachate to achieve a 40% to 60% moisture level and a temperature between 120xc2x0 F. and 140xc2x0 F. in steady state. One of the stated objectives of the Hudgins method, however, is to reduce the production of methane gas in the landfill. The Hudgins method thus does not provide a convenient way to produce methane for beneficial purposes.
The method of injecting steam into landfills according to this invention has several features, no single one of which is solely responsible for its desirable attributes. Without limiting the scope of this invention as expressed by the claims that follow, its more prominent features will now be discussed briefly. After considering this discussion, and particularly after reading the section entitled xe2x80x9cDetailed Description of the Drawings,xe2x80x9d one will understand how the features of this invention provide advantages, which include minimization of the amount of liquid introduced into the landfill, total moisturization and higher overall humidity of the landfill without the need to apply head pressure, promotion of settlement of the landfill, heating of the refuse, avoidance of clogging of gas extraction collectors, ability to distribute gaseous anaerobic fertilizer throughout the trash prism, increased methane production, and production of methane having higher Btu values as compared to methane produced in other landfills.
The present method comprises injecting steam into a landfill and collecting the methane produced by the decomposition/biodegradation of the organic component of the trash prism. The steam accelerates the decomposition of the organic refuse, thereby enhancing methane gas production by increasing the purity of the methane. By accelerating the decomposition of the organic refuse, the steam also increases the rate of settlement of the landfill. The time necessary to convert the landfill into property that is useful for purposes besides waste disposal is thus reduced. The reduced decomposition time also reduces the impact of the landfill on the environment.
The steam is derived from a source such as a boiler, heat exchanger or power plant, and is injected into the landfill through an array of steam injection wells. The methane is collected through an array of gas extraction collectors distributed throughout the landfill. The wells and collectors preferably comprise steel push-in screens and risers. The optimal location for the wells and collectors is preferably determined using a piezo-penetrometer test (PPT) profile, and the wells and collectors are preferably installed in the landfill using the PPT rig. The injectors and collectors can, however, also be installed with a drill rig.
Temperature and moisture sensors are preferably distributed throughout the landfill to monitor the conditions within the landfill. Feedback from these sensors enables the amount of steam injection to be adjusted to prevent liquid from accumulating within the landfill.
In a further aspect of the present method, both air and steam are injected into the landfill in order to maintain the landfill in the aerobic phase. Injecting only air into the landfill for aerobic degradation dries out the trash prism, which slows the decomposition process and may cause subterranean fires. Thus, the moisture lost in this process must be replaced. Introducing water is disadvantageous because it cools the refuse, slowing degradation, and travels downward due to gravity. Steam, however, warms the refuse because of its high temperature. The steam, which is a vapor, also travels in all directions within the landfill just as the injected air does. Further, the air that is injected is usually cool, especially in winter. Heating the air stream will prevent the air from cooling the interior of the landfill. Steam provides this heating action.
In a further aspect of the present method, the steam serves as a carrier medium for a gaseous anaerobic fertilizer, such as ammonia or ammonia nitrate. The steam may also serve as a carrier medium for a gas, such as nitrous oxide, that speeds the conversion of the landfill from the aerobic phase to the anaerobic phase.
In a further aspect of the present method, the temperature and pressure of the injected steam are raised to a level sufficient to melt the plastic component of the trash prism, thereby promoting further settlement of the landfill. The temperature and pressure are preferably raised after substantially all of the organic component of the refuse has decomposed. After substantially all of the plastic has melted, the gas extraction collectors preferably draw off the remaining steam in order to prevent condensation within the landfill.
In a further aspect of the method, the conditions within the landfill are preferably monitored using a PPT profile. The rate of settlement, and the rate of organic decomposition provide important information about the effectiveness of the present method.
In a further aspect of the method, the volume of the plastic component of the refuse is reduced prior to placing the refuse within the landfill. The plastic is preferably melted by placing the refuse in a containerized trammel and applying high-temperature and high-pressure steam. If the plastic is to be recycled, it is preferably removed from the refuse using screens.